I'd say the "undying" titles of later Wrath tiers. Those, as far as I know, could not be cheated in any way and prove that your group actually completed a pretty difficult and non-forgiving task.

I am speaking of:The Argent DefenderConqueror of UlduarChampion of Ulduar

The Immortal and The Undying are slightly below the line because although they got removed later on, you could massively outgear them already during Wrath.

Herald of the Titans used to be one of those, but as others pointed out, is now significantly easier with the stat inflation and new talents. But it's still a nice title and you DO need a group of friends willing to level up to 80 and get some decent gear.

Wrath realm-first titles can be a little bit tricky when they come from a terrible realm. I remember when one of our guilds got Death's Demise while their direct competitors were progressing heroic Lich King, so ... yeah. But despite of that, those titles are also pretty prestigious - probably with Grand Crusader on top (Anub'arak is a bitch no matter what gear you have), followed by Death's Demise and Celestial Defender.

Just would like to know the titles people find the most prestigious, for me personally it's Hand of Adal followed by Gladiator(legit ones atleast) also the Insane hardly see that one ever.

Hand of Adal, Herald of the Titans, Scarab Lord, the Immortal, the Undying, and Gladiator. Those aren't in any order. (For "Herald", I always check someone's feats of strength to make sure they got it legitimately)

Me personally, being a casual that is at most a LFR hero, so no (while current content) raiding or PvP titles?

The Insane & The Beloved.

Getting Loremaster and Seeker back in Wrath was also a pretty nice for me, both of which are obviously much easier now. (Seeker's 3k quests is pretty trivial 2 expacs later)

EK/Kalimdor was just a big lump sum of quests, and the hardest part was trying to figure out which ones you had completed already and what ones were still available to do. Plus so many that ended up being continent hopping ones, then certain ones in the chain not giving credit, etc. I got stuck on finishing Kalimdor's with about a dozen left to finish it, so much that I actually farmed up my full T0 and then did the entire T0.5 upgrade chain to get those final dozen or so. The Silithid Cultist farming part with the bad RNG not getting the Flame Duke to spawn... so much nerd rage!